1. Field of the Invention
This invention relates to an image pickup system comprising a light source, a camera, and a measured target located in an optical path therebetween and, more particularly, to a lock-in image pickup system for performing imaging by bringing a period of turning-on/off of the light source into sync with a frame of the camera.
2. Description of Related Art
As an image pickup system, a THz image pickup system in a terahertz (THz) frequency range is known.
A terahertz wave (a THz wave) refers to an electromagnetic wave having frequencies of about 0.1 to 10 THz (wavelengths of 3000 μm to 30 μm). That is, the terahertz wave (the THz wave) has wavelengths located between micro wave and an infrared, and is called a submillimeter wave. It is known that the terahertz wave (the THz wave) easily passes through nonconductive (nonmetal) materials such as papers (paper products), plastic, cloth (clothing), vinyl, ceramics, lumber, bones, teeth, fat, powder, dried foods, walls, or the like and nonpolarized materials. Therefore, for example, the terahertz wave (the THz wave) can be used to detect metallic or nonmetallic arms hided in the manner which is almost similar to a case where an X-ray is currently used to security in airport.
Inasmuch as the terahertz wave (the THz wave) easily passes through the papers (the paper products) in the manner which is described above, the terahertz wave (the THz wave) may be applicable, for example, to detect contents in an envelope without opening the envelope.
Referring now to FIG. 1, the description will proceed to a related terahertz image pickup system. The related terahertz image pickup system comprises a refrigerator 101, a quantum cascade laser 102, a first off-axis parabolic mirror 103, a second off-axis parabolic mirror 104, a target (a measured target) 105, a Si lens 106, an a micro bolometer camera 107 containing a (320×240) pixel micro bolometer array sensor 108. The example being illustrated shows a case where the target (the measured target) 105 comprises an envelope.
The quantum cascade laser 102 is contained in the refrigerator 101. The quantum cascade laser 102 serves as a THz light source and radiates an emission line (a THz wave) having a frequency of 3.1 THz. Radiated from the quantum cascade laser 102, the emission line (the THz wave) is reflected by the first off-axis parabolic mirror 103 and the second off-axis parabolic mirror 104 and passes through the target (the envelope) 105. Passed through the target (the envelope) 105, the THz wave is subjected to a parallel THz wave by the Si lens 106, and is picked up by the micro bolometer camera 107 to obtain a THz transmission image.
FIG. 2 show a view showing a method of acquiring a difference image in the related terahertz image pickup system illustrated in FIG. 1.
FIG. 3 shows the THz transmission image acquired by using the related terahertz image pickup system illustrated in FIG. 1. In FIG. 3, a reference numeral of 109 indicates a paper on which a letter of MIT is written with a pencil, and a reference numeral of 110 indicates the THz transmission image obtained by picking up the paper 109 put in the envelope 105.
Various prior art documents related to this invention are already known.
By way of illustration, JP 07-134800 A (which will later be called Patent Document 1 and which corresponds to U.S. Pat. No. 5,598,145) discloses a “driver image pickup device” in which a face image according to infrared radiation LED illuminating light only can be obtained by subtracting an “image according to extraneous light” from an “image according to the extraneous light plus LED turned on” in order to omit influence of the extraneous light.
In addition, JP 2003-208599 A (which will later be called Patent Document 2) discloses an “object recognition device” comprising: comparing an image in a case of turning on a luminaire with an image a case of turning out the luminaire; extracting information due to the luminaire alone from a difference therebetween; and creating an image corresponding to one picked up under a illumination condition within a constant range.
Furthermore, JP 2002-232907 A (which will later be called Patent Document 3) discloses a “digital image pickup device” comprising: subtracting, from first image data obtained on turning on a flash, second image data obtained on turning off the flash to calculate difference image data; and calculating, as illumination component data, data indicative of influence where an illumination environment to a subject provides to the image on the basis of the first image data, the second image data, the difference image data, and flash spectroscopic data. In the digital image pickup device disclosed in Patent Document 3, it is possible to further calculate suitable illumination component data and object color component data by obtaining plural set of the first images and the second images with exposure conditions such as a shutter speed, an iris value, and so on changed.
Inasmuch as the conventional examples disclosed in Patent Documents 1-3 calculate a difference image between one frame image upon turning on the light source and one frame image upon turning off the light source, it is disadvantageous in that image quality (a signal-to-noise ratio) is not as improved as one expected. This is because fixed background noises are removed by calculating the difference image but it is impossible to remove random noises.
In order to further improve the image quality, it is effective to a method of obtaining a plurality of frames including image data upon turning on the light source and image data upon turning off the light source, and carrying out average processing on the plurality of items of image data to decrease the random noises, as described in Patent Document 3. However, in a case where an operation processing is carried out by capturing the image data of the plurality of frames into an external storage and processing device such as a personal computer, it is feared that the image data is loss by failing the capturing.
In this event, as shown in FIG. 4, the image data of the frame upon turning on the light source may be erroneously used as the image data of the frame upon turning off the light source or the image data of the frame upon turning off the light source may be erroneously used as the image data of the frame upon turning on the light source. It is therefore disadvantageous in that the image quality is not improved although the average processing is carried out.
It may be considered to a method of carrying out the average processing by estimating whether captured image data corresponds to the frame upon turning on the light source or to the frame upon turning off the light source after capturing all of the image date into the external storage and processing device. However, this method is disadvantageous in that a time is consumed on the data processing and is disadvantageous in that the image quality is not improved in a case where the image quality is low because the accuracy of estimating becomes low.
Furthermore, when a sensor embedded in the camera has a large time constant, it is impossible to acquire stable image data at frames immediately after switching from turning-off of the light source to turning-on thereof or switching from turning-off of the light source to turning-off thereof. It is therefore impossible to improve the image quality except in cases where the average calculation is carried out by excluding the image data corresponding to those frames. However, it is difficult to accurately estimate whether or not those image date correspond to the frames immediately after switching turning-on/off of the light source after the image data are captured.